Peripheral Sensory Neuronal Fc-epsilon-Receptor â…  Plays A Role In Allergic Itch

Background and ObjectiveItch is an unpleasant sensation that elicits the desire or reflex to scratch, which may be caused by skin or systemic disease. Chronic pruritus is often hard to treat and is one of the health challenges worldwide. Similar with pain, itch sensation is originated mainly in peripheral unmyelinated C-fiber terminals, which can be directly or indirectly activated by pruritic mediators via specific neuronal receptors. The activated C fibers will transmit this information to the central nervous system, where it might cause the sensation of itch. Ocular or cutaneous pruritus is one of the most common symptoms of allergy, and currently available anti-allergic drugs are often ineffective or with annoying side effects. A common feature among allergic conditions is the elevated level of antigen-specific immunoglobulin (Ig) E which may form immune complex (IC) with the antigen. We previously demonstrated that IgG-IC directly excited a subpopulation of peripheral nociceptive neurons in the dorsal root ganglion (DRG) through the neuronal Fc-gamma receptor I (FcyRI). IC may directly induce hyperexcitability or even action potential discharges in nociceptive neurons. It was also found that the high-affinity activating receptor for IgE (FcεRI) was expressed in nociceptive DRG neurons. We hypothesize that IgE-IC may directly activate the FcεRI expressed in nociceptive sensory neurons in the trigeminal ganglion (TG). The purpose of this study was to explore a potential mechanism that FcεRI of peripheral nociceptive neurons may involve in the pathogenesis of ocular itch in a mouse model of allergic conjunctivitis.Materials and MethodA mouse model of allergic conjunctivitis was induced by sensitization with a specific antigen oval albumin (OVA) and an adjuvant (aluminum hydroxide). Upon successful sensitization, OVA solutions were applied to both eyes to induce ocular allergy and itch-related behaviors. IgE-IC was prepared by using the OVA as antigen and mouse anti-OVA IgE as antibody. Using a combination of behavioral, immunohistochemical (IHC), molecular and electrophysiological methodologies, we studied itch-related behaviors of the eyes in mice, the expression of FcεRs in the TG, and the effect of IgE-IC on the activation of TG sensory neurons in vivo and in vitro.1. Behavioral observation of ocular itch and pain.1.1. Predominantly painful substance was dripped to the eyes of mice and the pain-related behaviors were recorded.1.2. Predominantly pruritic substance was dripped to the eyes of mice and the itch-related behaviors were recorded.2. Induction of a mouse model of allergic conjunctivitis with ocular itch.2.1. OVA allergen was dripped to the eyes of sensitized mice to induce the mouse model of allergic conjunctivitis.2.2. The ocular itch behavior was accessed after dripping allergen to both eyes of sensitized or control mice.2.3. In separate experiments, mice were pre-treated with histamine HI receptor antagonist or mast cell stabilizer, or anti-FcεRI blocking antibody. The ocular itch behavior was accessed after allergen challenging.3. Detection of FcεRs expression in TG neurons by IHC, qPCR and Western Blotting in naive, sensitized and allergic mice.4. Calcium imaging was performed in acutely dissociated TG neurons to investigate the neuronal effects of different doses of IgE-IC, or antigen and IgE alone in vitro. Results1. Behavioral differentiation between itch and pain in the mouse eye.Topical application of 5 μM Capsaicin (predominantly painful substance) to the mouse eye induced bouts of wiping toward the eyes with the forelimb; while topical application of 10 μM BAM8-22 (predominantly pruritic substance) induced bouts of scratching toward the eyes with the hind limb.2. Antigen challenging to the eyes of sensitized mice induced itch-related behaviors which were partially independent to histamine or mast cell activation.2.1. Topical application of 0.1%,1% or 5% OVA (allergen) solution induced a dose-dependent increasing in the scratching behaviors toward the challenged eyes in sensitized but not control mice.2.2. The ocular itch-related behaviors, induced by allergen in the eyes of sensitized mice, cannot be totally blocked by pre-treatment of mast cell stabilizer or histamine I receptor antagonist.2.3. Pre-treatment with anti-FcεRI blocking antibody via topical application abolished the scratching behaviors induced by allergen challenging to sensitized mice eyes.3. Both the high-affinity IgE receptor FcεRⅠ (α,β and γ subunits) and the low-affinity IgE receptor CD23 (FcεRⅡ) were expressed in dissociated or intact TG neurons.4. Up-regulation of neuronal FcεRI in the TG of allergic mice.4.1. The protein levels of FcεRI a and β, but not γ subunits were upregulated in the TG of allergic mice.4.2. FcεRI a subunit was found co-expressed with CGRP (a marker for peptidergic nociceptive neurons) and IB4 (a marker for non-peptidergic nociceptive neurons) and was also expressed in the Mrgprd+or Mrgpra3+(itch-specific) neurons.5. IgE-IC activated dissociated TG neurons via neuronal FcεRI.5.1. IgE-IC can induce a dose-dependent [Ca2+]i increase in dissociated TG neurons.5.2. Application of a FcεRI-specific siRNA significantly reduced the expression of FcεRI in dissociated TG neurons.5.3. Compared with scrambled (control) siRNA, knockdown of FcεRI with a specific siRNA significantly reduced the percentage of TG neurons activated by IgE-IC as indicated by intracellular calcium increase.6. CD23 (FcεRII) was down-regulation in the TG of allergic mice.Conclusion1. Functional FcεRI and FcεRII were expressed in the nociceptive sensory neurons of TG.2. IgE-IC can activate dissociated TG neurons and induce Ca2+ increase via neuronal FcεRI.3. The protein levels of FcεRI were increased in the TG of allergic mice, which may enhance the neuronal sensitivity to IgE-IC.4. Direct activation of FcεRI on peripheral nociceptive (and tentative pruritic) sensory neurons by IgE-IC may play a role in the producing of ocular allergic itch which was partially independent to the mast cell activation or histamine release in the mice.5. Modulation of neuronal FcεRI and FcεRII expression may involve in the pathogenesis of allergic itch in mouse allergic model.Clinical implicationsOur research revealed a previously unknown mechanism underlying allergic itch: The high-affinity IgE receptor FcεRI in peripheral nociceptive (and tentative pruritic) sensory neurons could be directly activated by IgE-IC and may induce itch in allergic conditions. This finding may provide potentially novel strategies for the treatment of allergic itch.